Several mechanisms are known to provide for varying the power of a microscope. U.S. Pat. No. 2,103,525 discloses a binocular or stereo microscope with a multiple objective carrier which is movable along an arcuate track via a rack and pinion mechanism. U.S. Pat. No. 2,133,509 discloses a similar stereo microscope including an objective carrier and an inclined track. By moving the objective carrier vertically as well as laterally different objectives may be brought into alignment with the optical axis. The purpose of such an arrangement is to eliminate interference between the longer objectives and large specimens. However, the arrangement is such that the lower end of the track is always potentially in the way. Revolving nosepieces such as disclosed in U.S. Pat. Nos. 1,971,061 and 3,565,512 are well known and probably the most common method of providing for a selection of various magnifying powers. Attention is also directed to U.S. Pat. No. 2,237,943 which discloses a plurality of intermediate optical systems secured in a rotatable housing.